1. Field of the Invention
The present invention relates to a profiling controlling method and controller for contact type probes, and a contact type measuring machine. More particularly, it relates to a contact type probe profiling controlling method and controller capable of measuring a shape, roughness of a surface or the like of a soft object to be measured such as a plastic lens or aluminum product while leaving noncontact trace on the object, and a contact type measuring machine equipped with the contact type probe which is profiling-controlled by the controller.
2. Description of the Related Art
As a touch sensor for probes used in the case of measuring a fine surface shape of an object to be measured with a fine shape measuring machine or a surface roughness measuring machine, or measuring an inner shape of a hole with a small hole measuring machine, the applicant proposes an excitation type force sensor shown in FIG. 1, the sensor being disclosed in Japanese Published Unexamined Patent Application No. 2001-91206 (Patent document 1). In the force sensor 10, piezoelectric elements 20 is adhered to both surfaces of a metallic base 12 integrated with a stylus 14, the piezoelectric element being separated into an exciting electrode (referred to as an exciting piezoelectric element) 22 and a detecting electrode (referred to as a detecting piezoelectric element) 24. Additionally, a contact point 16 constituted by a diamond chip or ruby ball is fixed to a tip (lower end in FIG. 1) of the stylus 14.
An example of a relationship between input amplitude and output amplitude of the force sensor 10 is shown in FIG. 2. In the case where the contact point 16 is in noncontact with a work 8 which is an object to be measured, when a specified input amplitude Pi is applied to the exciting piezoelectric element 22, the stylus 14 vertically ultrasonically vibrates at an amplitude of, for example, several nm to several tens nm, and a signal of an output amplitude Po appears in the detecting piezoelectric element 24.
When the contact point 16 comes into contact with the work 8, as shown in FIG. 3, the magnitude of the output amplitude decreases from Po to Px. When an output amplitude ratio k0=(Px/Po)×100=90%, 135 [μN] is obtained as measurement force F in an example of a relationship between the output amplitude ratio k0 and the measurement force F. When the force sensor 10 connected to an actuator (not shown) is controlled so that the output amplitude ratio k0 is constantly kept, measurement force (F) fixed profiling measurement (referred to as force fixed profiling measurement) can be realized that is capable of measuring a shape or roughness of the work 8.
In the case of measuring the work with use of a contact type probe, as shown in FIG. 5, approach (operation of shifting the probe, at low speed, for contact with the object to be measured, detecting the contact, and stabilizing the contact by a target measurement force) to the work 8, which is the object to be measured, is required. Since, in such a force sensor 10, there occurs no measurement force unless the sensor comes into contact with the object to be measured, approach drive before contact with the force sensor and the object to be measured is normally performed by position control, etc., using a scale detector, and the position control is switched to force control after the contact with the sensor and the work 8. Then, the work 8 or probe (force sensor 10) attachment side is driven, and the sensor scans a surface of the work in a force control state. During the scan, a scale value of the probe and a position of the work are read as a shape measurement position. Then, after the force fixed profiling measurement ends, the force control is switched to the position control, and the sensor retracts to a retraction position.
During the approach operation, the target measurement force (force judged at which contact is stabilized) is compared, for detecting the contact, with the measurement force detected by the force sensor 10, and the position control is switched to the force control at the time when the measurement force exceeds the target measurement force. In this case, a delay time, e.g. about 100 μs, is caused to contact determination due to delay of an electric circuit, delay of calculation time of a digital controller, or the like. Consequently, as shown in FIG. 6, the probe is pushed into the work more deeply (over-push) than a position (measurement position) where the sensor is pushed into the work by the target measurement force. Further, when an approach speed is increased for raising measurement efficiency, not only does the amount of over-push become large but an impact in the contact become large, and, a contact trace remains on the object to be measured, which may cause problem especially in the case where the object is a lens or metal mold.
In order to prevent a contact trace from occurring due to approach, as shown in FIG. 7, a method has been conventionally employed that reduces, by reducing the approach speed as much as possible, the push-in amount, as much as possible, caused by the delay time in the contact determination and simultaneously reduces the impact in the contact, and thus prevents the contact trace from occurring on the object to be measured.
However, in the method, the approach speed must be reduced as much as possible, and the measurement efficiency is conspicuously lowered.